The present invention relates to a styrene resin composition which is excellent in sliding properties and which does not provide molded products having poor appearance during molding.
Considerable progress has recently been made in the technical developments in the fields of household appliances and office automation (OA) apparatuses and the rate of propagation thereof has greatly increased. In such fields of household appliances and OA apparatuses, metallic materials have been increasingly superseded by plastic materials accompanied by progress in mass production techniques and requirements for lighter and smaller apparatuses. Thus there have been used plastics as materials for not only housings of apparatuses but also for so-called mechanical parts such as gears, cams, pulleys, bearings and chassises.
In such fields, there has been principally used engineering plastics such as polyamides, polyacetals, polyester resins and polycarbonates which have excellent mechanical strength such as tenacity, wear and rub resistance, fatigue and creep resistance and rigidity.
It is also desireable to save cost and thus there have been many investigations to develop processing techniques such as a monolithic molding technique and to substitute cheaper plastics for the foregoing plastics. As a result, a portion of the engineering plastics have been replaced by widely used resins reinforced by glass fibers or carbon fibers, for instance ABS resins and PS resins.
It is well known in such industrial fields that a resin composition having self-lubricating properties can be obtained by incorporating silicone oil into a widely used resin such as a styrene resin and it has already been, put into practical use in a sliding part in which polyacetal resins have been mainly utilized, but such a resin composition is not yet always sufficient.
For instance, silicone oil-containing rubber-modified styrene resins as disclosed in Japanese Patent Un-examined Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 60-217254 are excellent in sliding properties, but cause various troubles during molding. More specifically, the amount of silicone oil to be incorporated into the styrene resin should be not less than 2 parts by weight, in most cases, to impart sliding properties required of such applications to the styrene resins. The compatibility between silicone oil and the rubber-modified styrene resins is substantially low and silicone oil is liable to cause separation from the rubber-modified resins within a mold and a molding machine during molding as the added amount thereof increases. This leads to the production of molded products having poor appearance such as the occurrence of silver streaks and burn spots.
To solve this problem, it has been tried to lengthen the molding cycle or to reduce the molding temperature. However, these methods cannot substantially solve the problem since many processes are required for screening the resultant molded products and thus it has been strongly desired to solve the problem as soon as possible.
Furthermore, J.P. KOKAI No. Sho 59-108016 discloses a method for improving the dispersibility of silicone oil in a styrene polymer by adding silicone oil at a specified time in the course of polymerization when a styrene monomer is polymerized in the presence of a rubbery substance, but this method also does not solve the foregoing problem.
In addition, as disclosed in J.P. KOKAI Nos. Sho 62-39610 and Sho 63-182361, it is suggested that the aforementioned problem can be solved if the content of silicone oil in the styrene resins is reduced by simultaneously using silicone oil and other sliding property-improving agents. However, the molded products resulting from such resin compositions are easily peeled off in thin layers at portions adjacent the mold gate and thus the problem has not yet completely been solved.